1. Field of Invention
The present invention generally relates to fiber-grating-based sensing systems, and more particularly to a distributed fiber-grating-based sensing system capable of accurately measuring wavelengths.
2. Description of Related Art
Fiber Bragg gratings have been used to detect variations of physical parameters, such as temperature or strain, from where the fiber gratings are located due to the fact that changes in the environmental parameters generally result in changes of center wavelength of the light reflected from the fiber Bragg gratings. By measuring the center wavelength of the reflected light from the fiber gratings, the environment parameters can be determined.
One of the most frequently-used methods to detect the center wavelength of the reflected light from the fiber gratings is to provide a broadband light source or a tunable laser beam incident to a fiber grating, and measure optical characteristics such as the center wavelength, intensity, and time delay of the narrow-band light reflected from the fiber grating. If multiple fiber gratings are used, a distributed multiple-points detection can be achieved by detecting reflected lights with different wavelength from different fiber gratings.
To detect a center wavelength of the narrow-band light reflected from the fiber gratings, there are mainly three approaches: (1). Using unbalanced Mach-Zehnder interferometer. (2). Using tunable Fabry-Perot filter. (3). Using fiber grating elements. The first two approaches require a reference light source to ascertain the absolute optical wavelength, due to the fact that both the Mach-Zehnder interferometer and tunable Fabry-Perot filter are susceptible to environmental changes and can easily result in wavelength shifts while measuring. The reference light source can be temperature-compensated fiber gratings or fixed Fabry-Perot filters.
Conventional fiber-grating-based sensing systems can be constructed by applying the above-mentioned technologies, which are shown in FIG. 1 and FIG. 2. Unfortunately, there are disadvantages found in the systems shown in FIGS. 1 and 2. (1). The tunable Fabry-Perot filters are quite expensive and scanning speed is not fast enough. (2). Integrated optics unbalanced Mach-Zehnder interferometers are too expensive, while fiber unbalanced Mach-Zehnder interferometers have problems accurately controlling the difference of the optical path between two branches. Furthermore, they are difficult to fabricate, and the random shift of the phase difference is significant. (3). There is a problem of wavelength ambiguity by using the Mach-Zehnder interferometer to detect multiple fiber gratings.